1. Technical Field
The present invention is related to fiber amplifiers, and more particularly to packaging for double cladding fiber amplifiers.
2. Description of Related Art
Double cladding fiber structures (e.g., as shown in U.S. Pat. No. 4,815,079, xe2x80x9cOptical fiber lasers and amplifiersxe2x80x9d) have been demonstrated to be an effective approach (e.g., as shown in L. Goldberg, J. P. Koplow, D. Kliner, xe2x80x9cHighly efficient 4-W Yb-doped fiber amplifier pumped by a broad stripe laser diodexe2x80x9d, Optics Lett., v. 15, pp. 673-675, 1999) for implementing high power fiber lasers and amplifiers. In a double cladding fiber, a single mode doped core is surrounded by a large (typically 100-200 xcexcm), multimode inner cladding, which is in turn surrounded by an outer cladding. The pump light propagates in the large, multimode inner cladding and is gradually absorbed by the active dopant in the fiber core. Since double cladding fibers are typically pumped by non-diffraction limited broad stripe lasers diodes, maximizing of the inner cladding NA makes it possible to use a larger angular cone of incident pump light. This in turn allows the of use larger de-magnification factors for focusing the pump source onto the fiber, and the use of broad stripe laser diodes with a larger stripe widths and output power. The inner cladding numerical aperture is given by NA=n{square root over (2xcex94n)} where n=1.46 is the refractive index of a silica inner cladding, and xcex94n≅(nxe2x88x92nclad)/n is the normalized index difference between the inner and outer cladding.
Using polymers as the outer cladding material, refractive index of 1.34 (Teflon) to 1.41 (Silicone) are possible, resulting in an inner cladding numerical aperture in the range of NA=0.60 to 0.38. Double cladding fibers can also be fabricated with a glass outer cladding, resulting in an all-glass fiber structure (except for a polymer outer protective jacket). A refractive index difference between the silica inner cladding and the outer cladding is obtained by using a glass with a different composition and lower refractive index for the outer cladding. With fluorinated silica glass outer cladding, numerical apertures up to 0.25 can be achieved for the inner cladding.
An effective method for coupling the emission of a broad area pump diode into the inner cladding of a double cladding fiber is through a v-groove (e.g., as shown in U.S. Pat. No. 5,854,865, xe2x80x9cMethod and Apparatus for Side-Pumping of Optical Fiberxe2x80x9d) fabricated into the fiber. The method makes it possible to couple the pump light directly into the fiber with high efficiency. In one implementation, shown in FIGS. 1A-1B, the emission of a broad stripe laser diode is collected by a micro-lens and then focused onto the v-groove facet. When the incident pump light is confined, in the yz plane (see FIGS. 1A-1B), to angles ranging from xe2x88x922.6xc2x0 relative the vertical to an angle of arcsin(NA) on the other side of the vertical, the light is reflected by total internal reflection at the v-groove facet and couples into the inner cladding of the fiber. The positive angle limit above represents the maximum propagation angle allowed by the numerical aperture of the inner cladding, and varies from 22xc2x0 for NA=0.38 to 37xc2x0 for NA=0.6, both measured in air. Using the v-groove technique, a total diode-to-fiber coupling efficiency of up to 90% can be achieved.
Alternatively, v-groove coupling can be implemented without any lenses between the diode and the fiber (e.g., as shown in L. Goldberg, J. Pinto, xe2x80x9cDouble cladding fiber amplifiers with lens-less side-pumping,xe2x80x9d paper CFC1, CLEO, 2000). In the lens-less coupling arrangement, the light from the pump diode is allowed to diffract freely and is intercepted by both v-groove facets resulting in coupling of pump light into both sides of the double cladding fiber.
In the prior art, when a conventional double cladding fiber with a polymer outer cladding is used, a hermetic seal can not be achieved since the polymer is permeable to moisture. This polymer can not be removed in the section inside the ferrule since any metallization of the inner cladding fiber surface would result in absorbing the pump light propagating in the inner cladding of the fiber.
An advantage of the present invention is that it overcomes the disadvantages and shortcomings of the prior art. A novel method is disclosed for constructing high power fiber amplifiers with hermetically sealed laser diode pump module. The amplifier uses double cladding fibers (e.g., as shown in U.S. Pat. No. 4,815,079, xe2x80x9cOptical Fiber Lasers and Amplifiersxe2x80x9d), and a v-groove side-pumping technique (e.g., as shown in U.S. Pat. No. 5,854,865, xe2x80x9cMethod and Apparatus for Side-Pumping of Optical Fiberxe2x80x9d) that directly couples the pump light into the fiber. Another advantage of the present invention is that it provides a method and apparatus for hermetically sealing the pump diode by using a double cladding fiber with an all-glass construction is described. In a double cladding fiber with an all-glass construction, the outer cladding is preferably made of glass, allowing a hermetic seal to be created between the outer fiber surface and the inside surface of ferrule attached to the hermetic package. To form a hermetic seal, the double cladding fiber is preferably metal-coated and then soldered inside the ferrule.
Yet another advantage of the present invention is that it achieves high efficiency of coupling the pump diode into the fiber. This advantage may be achieved due to the use of a microlens to collect the light from the laser diode and re-focus it onto the surface of the v-groove at a divergence that is less than the relatively low numerical aperture of the inner cladding in an all-glass double cladding fiber.
Another advantage of a hermetic package according to an embodiment of the present invention is the use of a removable ferrule that allows both sides of the double cladding fiber to be inserted into ferrules attached to the module enclosure. An additional advantage of the present invention is the use of a detachable ferrule that allows the assembly of a pump module with a continuous length of fiber pigtails on both sides of the module.
Still another advantage of the present invention is that it allows hermetic packaging of double cladding pump modules. This advantage may be achieved by the use an all glass double cladding fiber to allow a hermetic seal to be formed by soldering the metallized fiber inside ferrules. Another advantage of the present invention is that it provides a method of assembling the key components of the amplifier, including the pump diode, the microlens and the fiber with the v-groove, inside the hermetic enclosure. Yet another advantage of the invention is that it provides a means of focusing the pump light into the all glass double cladding fiber with an angular divergence cone which is less that the 0.25 numerical aperture of the fiber inner cladding.
These and other advantages are achieved by a hermetic pump module for coupling light from a pump source into an optical waveguide. The hermetic pump module includes a hermetically sealed housing, wherein the hermetically sealed housing contains an all-glass double cladded fiber with an outer cladding, an inner cladding, and a core, wherein the double cladded fiber includes a v-groove that extends through the outer cladding into the inner cladding, a pump source that emits a light, and a transparent substrate, bonded to the glass outer cladding with a transparent adhesive, wherein the light passes through the transparent substrate and into the all-glass double cladded fiber and the v-groove couples the light into the all-glass double cladded fiber. The substrate may be anti-reflection (AR) coated on the pump source side to reduce Fresnell reflection.
Alternatively, these and other advantages are achieved by a hermetic pump module, without a substrate, for coupling light from a pump source into an optical waveguide. The hermetic pump module includes a hermetically sealed housing, wherein the hermetically sealed housing contains an all-glass double cladded fiber with an outer cladding, an inner cladding, and a core, wherein the double cladded fiber includes a v-groove that extends through the outer cladding into the inner cladding, and a pump source that emits a light. Use of a substrate is eliminated in this embodiment of the hermetic pump module and instead light from the pump source is coupled through a side-wall of the fiber directly into the fiber by the v-groove. As with the substrate in the previous embodiment above, the side-wall of the fiber may be AR coated to reduce Fresnell reflection. These and other advantages are also achieved by a method of assembling a hermetic pump module that couples light from a pump source into an optical waveguide. The method includes the steps of coarsely positing a microlens mount inside a hermetic housing, fabricating a v-groove in an all-glass double cladded fiber, bonding the all-glass double cladded fiber to a fiber mount, precisely positioning the microlens holder and the fiber mount relative to each other and a diode in the hermetic housing to maximize pump coupling of light emitted from the diode onto the v-groove and into the all-glass double cladded fiber, and covering the hermetic housing in order to hermetically seal the hermetic housing. The microlens mount includes a microlens.